Photomultiplier tube optimized for surface inspection in the ultraviolet

Abstract

Disclosed herein is a PhotoMultiplier Tube (PMT) designed for use with a surface inspection system such as the Surfscan system, which operates at 266 nm wavelength. The inventive PMT is high efficiency, low noise, and low gain, a combination of features that is specific to the application and contrary to the features of PMT's in the art. The inventive PMT is designed to be tuned to a specific narrow band wavelength of incident light, thereby optimizing the QE at that wavelength. It is further designed to combine a small number of dynodes each having substantially higher secondary electron gain than typical dynodes. By designing the PMT in this way, the excess noise factor is dramatically reduced, yielding a much improved S/N, while still maintaining the overall PMT gain in the lower range suitable for use in a surface inspection system.

With this in mind, we claim: 
     
       1. A photomultiplier tube comprising:
 a photocathode composed of high quantum efficiency GaN in reflection mode; 
 a number of secondary electron gain dynodes between two and four, said dynodes being composed of CVD diamond; 
 a glass envelope surrounding said photocathode and dynodes, said glass envelope having an entry window coated with an antireflection coating optimized for 266 nm light; 
 wherein said photomultiplier tube has low gain and low noise, and has a high quantum efficiency at UV light of wavelength 266 nm. 
 
     
     
       2. A photomultiplier tube comprising:
 a photocathode composed of a bialkili material in reflection mode; 
 a number of secondary electron gain dynodes between two and four, said dynodes being composed of CVD diamond; 
 a glass envelope surrounding said photocathode and dynodes, said glass envelope having an entry window coated with an antireflection coating optimized for 266 nm light; 
 wherein said photomultiplier tube has low gain and low noise, and has a high quantum efficiency at UV light of wavelength 266 nm. 
 
     
     
       3. A wafer surface scanning inspection system including a photomultiplier tube as in  claim 1 . 
     
     
       4. A wafer surface scanning inspection system including a photomultiplier tube as in  claim 2 .